Want to build strength and lean muscle mass? Try creatine. A group of Canadian researchers conducted an eight-week study and found that insulin-like growth factor-1 (IGF-I), which is known to produce anabolic effects in adults, increased by more than 20% among participants who supplemented with creatine and performed resistance-training exercises. Just be sure not to take excessive amounts, as various side effects have been reported. According to the European Food Safety Authority , creatine supplementation of three grams per day is a safe amount.